Mri conditionally safe lead with multi-layer conductor

ABSTRACT

An implantable medical lead exhibits reduced heating under MRI conditions. The lead includes a multi-layer coil conductor including an inner coil layer, a middle coil layer disposed around the inner coil layer, and an outer coil layer disposed around the middle coil layer. Each of the coil layers is characterized by one or more of a filar thickness, a coil pitch, or a coil diameter configured such that the coil conductor exhibits a high inductance when exposed to MRI radiation. Each of the coil layers is electrically connected to the other coil layers to provide parallel conductive paths resulting in a coil conductor resistance suitable for defibrillation lead applications.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Provisional Application No.61/291,557, filed Dec. 31, 2009, which is herein incorporated byreference in its entirety.

TECHNICAL FIELD

Various embodiments of the present invention generally relate toimplantable medical devices. More specifically, embodiments of thepresent invention relate to MRI conditionally safe lead conductors.

BACKGROUND

When functioning properly, the human heart maintains its own intrinsicrhythm and is capable of pumping adequate blood throughout the body'scirculatory system. However, some individuals have irregular cardiacrhythms, referred to as cardiac arrhythmias, which can result indiminished blood circulation and cardiac output. One manner of treatingcardiac arrhythmias includes the use of a pulse generator (PG) such as apacemaker, an implantable cardioverter defibrillator (ICD), or a cardiacresynchronization (CRT) device. Such devices are typically coupled to anumber of conductive leads having one or more electrodes that can beused to deliver pacing therapy and/or electrical shocks to the heart. Inatrioventricular (AV) pacing, for example, the leads are usuallypositioned in a ventricle and atrium of the heart, and are attached vialead terminal pins to a pacemaker or defibrillator which is implantedpectorally or in the abdomen.

Magnetic resonance imaging (MRI) is a non-invasive imaging procedurethat utilizes nuclear magnetic resonance techniques to render imageswithin a patient's body. Typically, MRI systems employ the use of amagnetic coil having a magnetic field strength of between about 0.2 to 3Teslas. During the procedure, the body tissue is briefly exposed toradio frequency (RF) pulses of electromagnetic energy in a planeperpendicular to the magnetic field. The resultant electromagneticenergy from these pulses can be used to image the body tissue bymeasuring the relaxation properties of the excited atomic nuclei in thetissue. In some cases, imaging a patient's chest area may be clinicallyadvantageous. In a chest MRI procedure, implanted pulse generators andleads may also be exposed to the applied electromagnetic fields.

SUMMARY

The various embodiments of the present invention relate to implantablemedical electrical leads including multi-layer conductor coilsconfigured to minimize heat rise when exposed to MRI radiation.

In Example 1, an implantable electrical lead, comprising a flexiblebody, a connector assembly, an electrode, and a multi-layer coilconductor. The flexible body has a proximal region including a proximalend, a distal region, and a longitudinal body lumen extending from theproximal end through the distal region. The connector assembly isconfigured to mechanically and electrically connect the implantableelectrical lead to an implantable pulse generator. The electrode iscoupled to the flexible body in the distal region, and the multi-layercoil conductor extends within the longitudinal body lumen from theconnector assembly to at least the electrode. The multi-layer coilconductor includes a first coil layer including one or more filars woundso as to have a close pitch, and a second coil layer disposed about thefirst coil layer including one or more filars wound so as to have aclose pitch. The filars of the second coil layer are wound in the samepitch direction as the filars of the first coil layer. The first andsecond coil layers are electrically coupled to one another andconfigured to provide parallel conductive paths between the connectorassembly and the electrode such that the multi-layer coil conductor hasa maximum DC resistance of about 3 to 3.5 ohms.

In Example 2, the implantable electrical lead of Example 1, wherein thefilars of the first coil layer and the second coil layer areindividually electrically insulated.

In Example 3, the implantable electrical lead of Examples 1 or 2,wherein the one or more filars of the first coil layer are electricallyuninsulated from one another, and wherein the one or more filars of thesecond coil layer are electrically uninsulated from one another, and thelead further includes a layer of insulating material at least partiallydisposed between the first and second coil layers.

In Example 4, the implantable electrical lead of any of Examples 1through 3, wherein the one or more filars of the first and second coillayers are made from drawn filled tube (DFT) wires including a silvercore and an outer cladding of a cobalt-nickel-molybdenum alloy.

In Example 5, the implantable lead of any of Examples 1 through 4,wherein the filars of the first coil layer have a maximum filarthickness of about 0.001 inches, and wherein the filars of the secondcoil layer have a maximum filar thickness of about 0.003 inches.

In Example 6, the implantable electrical lead of any of Examples 1through 5, further comprising a third coil layer disposed about thesecond coil layer including one or more filars wound in a close pitchand in the same pitch direction as the filars of the first and thesecond coil layers.

In Example 7, the implantable lead of any of Examples 1 through 6,wherein the third coil layer has a maximum filar thickness of about0.004 inches.

In Example 8, the implantable electrical lead of any of Examples 1through 7, wherein an inductance of the first coil layer, the secondcoil layer and the third coil layer are different.

In Example 9, the implantable electrical lead of any of Examples 1through 8, wherein the first coil layer and the second coil layer have a1-filar, 2-filar, 3-filar, or 4-filar construction.

In Example 10, the implantable electrical lead of any of Examples 1through 9, wherein the first coil layer has a filar thickness ofbetween, and including, 0.001 inches and 0.002 inches.

In Example 11, the implantable electrical lead of any of Examples 1through 9, wherein the first coil layer is formed from a 0.0007 inchthick drawn filled tube (DFT) wire with a 1 to 4 filar construction, andthe outside diameter of the first coil layer is less than 0.004 inches.

In Example 12, the implantable electrical lead of any of Examples 1through 11, wherein the second coil layer has a filar thickness between,and including, 0.0007 inches to 0.003 inches.

In Example 13, an implantable medical lead, comprising a flexible body,a connector assembly, an electrode, and a multi-layer coil conductor.The flexible body has a proximal region including a proximal end, adistal region, and a longitudinal body lumen extending from the proximalend through the distal region. The connector assembly is configured tomechanically and electrically connect the implantable medical lead to animplantable pulse generator. The electrode is coupled to the flexiblebody in the distal region, and the multi-layer coil conductor extendswithin the longitudinal body lumen from the connector assembly to atleast the electrode. The multi-layer coil conductor includes an innercoil conductor having an outer diameter (OD), a close pitch, and aninner coil pitch direction of clockwise or counterclockwise. Themulti-layer coil conductor further includes a middle coil conductorhaving a close pitch, a middle coil pitch direction the same as theinner coil pitch direction, an inner diameter (ID) greater than the ODof the inner coil conductor, wherein the middle coil conductor radiallysurrounds the inner coil conductor along the length of the inner coilconductor. The multi-layer coil conductor also includes an outer coilconductor radially surrounding the middle coil with a close pitch and anouter coil pitch direction the same as the inner coil pitch direction.The inner coil conductor, the middle coil conductor, and the outer coilconductor are electrically coupled in parallel resulting in a DCresistance of approximately 3.5 ohms.

In Example 14, the implantable medical lead of Example 13, wherein themedical lead further includes one or more layers of insulating materialsurrounding one or more of the inner coil conductor, the middle coilconductor, or the outer coil conductor.

In Example 15, the implantable medical lead of Example 13 or 14, whereinthe inner coil conductor, the middle coil conductor, or the outer coilconductor have a 1 to 4 filar construction and each having a differentpitch.

In Example 16, the implantable medical lead of any of Examples 13through 15, wherein the inner coil conductor has a wire diameter of0.001 inches or less.

In Example 17, the implantable medical lead of any of Examples 13through 16, wherein the inner coil conductor is formed from a 0.0007inches drawn filled tube (DFT) wire with a 1-4 filar assembly and an ODof less than 0.004 inches.

In Example 18, the implantable medical lead of any of Examples 13through 17, wherein the outer coil conductor has a wire diameterbetween, or including, 0.0007 inches to 0.003 inches.

In Example 19, the implantable medical lead of any of Examples 13through 18, wherein each of the filars of the inner coil layer, themiddle coil layer, and the outer coil layer is individually insulated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a medical system including an MRIscanner, and an implantable cardiac rhythm management system implantedwithin a torso of a human patient according to various embodiments ofthe present invention;

FIG. 2A is a schematic view of an illustrative pulse generator and leadimplanted within the body of a patient which may be used in accordancewith some embodiments of the present invention;

FIG. 2B is a schematic view showing a simplified equivalence circuit forthe lead of FIG. 2A;

FIG. 3 is a schematic view illustrating an exemplary lead that may beused in accordance with one or more embodiments of the presentinvention;

FIG. 4 is a transverse cross-sectional view of the lead of FIG. 3 takenalong the line 4-4 in FIG. 3; and

FIG. 5 illustrates invention detail of a multi-layer coil conductorutilized in the lead of FIG. 3 according to an embodiment of the presentinvention.

The drawings have not necessarily been drawn to scale. For example, thedimensions of some of the elements in the figures may be expanded orreduced to help improve the understanding of the embodiments of thepresent invention. While the invention is amenable to variousmodifications and alternative forms, specific embodiments have beenshown by way of example in the drawings and are described in detailbelow. The intention, however, is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION

As explained in further detail below, various embodiments of the presentinvention relate to cardiac rhythm management (CRM) systemsincorporating new lead designs advantageously adapted for operation in amagnetic resonance imaging (MRI) environment. In some embodiments, theleads including conductor designs configured to provide suitableelectrical performance for delivering pacing and/or defibrillation shocktherapy and to minimize the lead's reaction to applied electromagneticenergy during MRI procedures.

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of embodiments of the present invention. It will beapparent, however, to one skilled in the art that embodiments of thepresent invention may be practiced without some of these specificdetails.

While, for convenience, some embodiments are described with reference toIMDs in the presence of MRI scanners, embodiments of the presentinvention may be applicable to various other physiological measurements,treatments, IMD devices, lead types, and other non-invasive examinationtechniques in which conductive leads are exposed to time varyingmagnetic fields. As such, the applications discussed herein are notintended to be limiting, but instead exemplary. In addition, variousembodiments are applicable to all levels of sensory devices from asingle IMD with a sensor to large networks of sensory devices.

FIG. 1 is a schematic illustration of a medical system 100 including anMRI scanner 110, an implantable cardiac rhythm management (CRM) system115 implanted within a torso of a human patient 120, and one or moreexternal device(s) 130 according to various embodiments. The externaldevice(s) 130 are capable of communicating with the CRM system 115implanted within the patient 120. In the embodiment shown in FIG. 1, theCRM system 115 includes a pulse generator (PG) 140 and a lead 150.During normal device operation, the PG 140 is configured to deliverelectrical therapeutic stimuli to the patient's heart 160 for providingtachycardia ventricular fibrillation, anti-bradycardia pacing,anti-tachycardia pacing, cardiac resynchronization therapy, and/or othertypes of therapy.

Thus, in the illustrated embodiment, the PG 140 can be an implantabledevice such as a pacemaker, an ICD, a cardiac resynchronization therapy(CRT) device, a CRT device with defibrillation capabilities (a CRT-Ddevice), or a comparable device. The PG 140 can be implantedsubcutaneously within the body, typically at a location such as in thepatient's chest. In some embodiments, PG 140 can be implanted in or nearthe abdomen.

The external device(s) 130 may be a local or remote terminal or otherdevice (e.g., a computing device and/or programming device), operable tocommunicate with the PG 140 from a location outside of the patient'sbody. According to various embodiments, external device 130 can be anydevice external to the patient's body that is telemetry enabled andcapable of communicating with the PG 140. Examples of external devicescan include, but are not limited to, programmers (PRM), in-homemonitoring devices, personal computers with telemetry devices, MRIscanner with a telemetry device, manufacturing test equipment, or wands.In some embodiments, the PG 140 communicates with the remote terminal130 via a wireless communication interface. Examples of wirelesscommunication interfaces can include, but are not limited to, radiofrequency (RF), inductive, and acoustic telemetry interfaces.

FIG. 2A is a more detailed schematic view of the CRM system 115including the illustrative PG 140 equipped with the lead 150 implantedwithin the body of a patient. In the embodiments depicted, CRM system115 includes PG 140 implanted near the patient's heart 160 and lead 150having a distal portion implanted with the patient's heart 160. As canbe seen in FIG. 2A, the heart 160 includes a right atrium 210, a rightventricle 220, a left atrium 230, and a left ventricle 240.

The lead 150 has a flexible body 200 including a proximal region 205 anda distal region 250. As shown, the lead 150 is coupled to the PG 140,and the distal region 250 of the lead body 200 is at least partiallyimplanted at a desired location within the right ventricle 220. Asfurther shown, the lead 150 includes a pair of coil electrodes 255, 257along the distal region 250, such that when implanted as shown in FIG.2A, they are positioned within the right ventricle 220 and right atrium210, respectively. As explained and illustrated in further detail below,the lead 150 includes one or more electrical conductors within the leadbody 200 (not visible in FIG. 2A) electrically coupling the electrodes255, 257 to circuitry and other electrical components within the PG 140for transmitting intrinsic cardiac signals from the heart 160 to the PG140 and also for transmitting electrical shocks or low-voltage pacingstimuli to the heart 160 via the electrodes 255, 257 or additionalelectrodes (not shown in FIG. 2A).

Although the illustrative embodiment depicts only a single lead 150inserted into the patient's heart 160, in other embodiments multipleleads can be utilized so as to electrically stimulate other areas of theheart 160. In some embodiments, for example, the distal portion of asecond lead (not shown) may be implanted in the right atrium 210. Inaddition, another lead may be implanted within the coronary venoussystem to facilitate pacing the left ventricle, i.e., in a CRT or CRT-Dsystem providing bi-ventricular pacing, as is known in the art. Othertypes of leads such as epicardial leads may also be utilized in additionto, or in lieu of, the lead 150 depicted in FIGS. 1-2. In short, thevarious embodiments of the present invention contemplate any multi-leadcombinations and configurations for use in CRM systems 115, whether nowknown or later developed.

During operation, the lead 150 conveys electrical signals between theheart 160 and the PG 140. For example, in those embodiments where the PG140 has pacing capabilities, the lead 150 can be utilized to deliverelectrical therapeutic stimulus for pacing the heart 160. In thoseembodiments where the PG 140 is an ICD, the lead 150 can be utilized todeliver high voltage electric shocks to the heart 160 via the electrodes255, 257 in response to an event such as a ventricular fibrillation.

As explained in detail below, various embodiments of the presentinvention relate to new lead designs that allow for improved mechanicalcharacteristics and safe operation in an MRI environment. In someembodiments, traditional conductor cables are replaced with low profilemulti-layer coil conductors. The multi-layer coil conductors allow leaddesigners to maintain a small lead profile in pace/sense leadapplications while better controlling axial elongation of the lead undertensile load.

FIG. 2B is a schematic view showing a simplified equivalence circuit 260for the lead 150 of FIG. 2A, representing the RF energy picked up on thelead 150 from RF electromagnetic energy produced by an MRI scanner. Asshown in FIG. 2B, voltage (Vi) 265 in the circuit 260 represents anequivalent source of energy picked up by the lead 150 from the MRIscanner. During magnetic resonance imaging, the length of the lead 150functions similar to an antenna, receiving the RF energy that istransmitted into the body from the MRI scanner. Voltage (Vi) 265 in FIG.2B may represent, for example, the resultant voltage received by thelead 150 from the RF energy. The RF energy picked up by the lead 150 mayresult, for example, from the rotating RF magnetic field produced by anMRI scanner, which generates an electric field in the planeperpendicular to the rotating magnetic field vector in conductivetissues. The tangential components of these electric fields along thelength of the lead 150 couple to the lead 150. The voltage (Vi) 265 isthus equal to the integration of the tangential electric field (i.e.,the line integral of the electric field) along the length of the lead150.

The ZI parameter 270 in the circuit 260 represents the equivalentimpedance exhibited by the lead 150 at the RF frequency of the MRIscanner. The impedance value ZI 270 may represent, for example, theinductance or the equivalent impedance resulting from the parallelinductance and the coil turn by turn capacitance exhibited by the lead150 at an RF frequency of 64 MHz for a 1.5 Tesla MRI scanner, or at anRF frequency of 128 MHz for a 3 Tesla MRI scanner. The impedance ZI ofthe lead 150 is a complex quantity having a real part (i.e., resistance)and an imaginary part (i.e., reactance).

Zb 275 in the circuit 260 may represent the impedance of the body tissueat the point of lead contact. Zc 280, in turn, may represent thecapacitive coupling of the lead 150 to surrounding body tissue along thelength of the lead 150, which may provide a path for the high frequencycurrent (energy) to leak into the surrounding tissue at the RF frequencyof the MRI scanner. Minimizing the absorbed energy (represented bysource Vi 265) reduces the energy that is transferred to the body tissueat the point of lead contact with the body tissue.

As can be further seen in FIG. 2B, the lead 150 has some amount ofleakage into the surrounding tissue at the RF frequency of the MRIscanner. As further indicated by 275, there is also an impedance at thepoint of contact of the lead electrodes 255, 257 to the surrounding bodytissue within the heart 160. The resulting voltage Vb delivered to thebody tissue may be related by the following formula:

Vb=ViZbe/(Zbe+ZI), where Zbe=Zb in parallel with Zc.

The temperature at the tip of the lead 150 where contact is typicallymade to the surrounding tissue is related in part to the powerdissipated at 275 (i.e., at “Zb”), which, in turn, is related to thesquare of Vb. To minimize temperature rises resulting from the powerdissipated at 275, it is thus desirable to minimize Vi (265) and Zc(280) while also maximizing the impedance ZI (270) of the lead 150. Insome embodiments, the impedance ZI (270) of the lead 150 can beincreased at the RF frequency of the MRI scanner, which aids in reducingthe energy dissipated into the surrounding body tissue at the point ofcontact 275.

In the various embodiments described in further detail below, theimpedance of the lead 150 can be increased by adding inductance to thelead 150 and/or by a suitable construction technique. For example, invarious embodiments, the inductance of the lead 150 is increased byselectively configuring the conductors used to supply electrical energyto the electrodes 255, 257.

FIG. 3 illustrates in further detail the exemplary lead 150 that may beused in accordance with one or more embodiments of the presentinvention. As shown in FIG. 3, the lead body 200 includes a proximal end305, and the lead 150 further includes a connector assembly 310 coupledto the proximal end 305 of the lead body, the coil electrodes 255, 257,and a tip electrode 312 which operates in the illustrated embodiment asa pace/sense electrode. Depending on the functional requirements of thePG 140 (see FIG. 1), and the therapeutic needs of the patient, thedistal region 250 of the lead 150 may include additional electrodes. Forexample, in some embodiments, the pair of coil electrodes 255, 257 canbe used to function as shocking electrodes for providing adefibrillation shock to the heart 160. In some embodiments, the lead 150can include a low-voltage (e.g., ring) electrode proximal to the distaltip of the lead 150 which is also operable as a pace/sense electrode,which can be included in addition to, or in lieu of, the tip electrode312. In short, the lead 150 can incorporate any number of electrodeconfigurations within the scope of the embodiments of the presentinvention.

In the illustrated embodiment, the connector assembly 310 includes aconnector body 320 and a terminal pin 325. The connector assembly 310 iscoupled to the lead body 200 and can be configured to mechanically andelectrically couple the lead to a header on PG 140 (see FIG. 1 and FIG.2). In various embodiments, the terminal pin 325 extends proximally fromthe connector body 320 and in some embodiments is coupled to an innerconductor (not shown in FIG. 3) that extends longitudinally through thelead body 200 to the tip electrode 312. In some embodiments, theterminal pin 325 can include an aperture extending therethroughcommunicating with a lumen defined by the inner conductor coil in orderto accommodate a guide wire or an insertion stylet.

In various embodiments, the tip electrode 312 is in the form of anelectrically active fixation helix at the distal end of the lead 150. Insome embodiments, the tip electrode 312 can be an extendable/retractablehelix supported by a mechanism to facilitate longitudinal translation ofthe helix relative to the lead body as the helix is rotated. In thoseembodiments, the terminal pin 325 may be rotatable relative to theconnector body 320 and the lead body 200 such that rotation of theterminal pin 325 relative to the lead body 200 causes the innerconductor, and in turn, the helical tip electrode to rotate andtranslate longitudinally relative to the lead body 200. Variousmechanisms and techniques for providing extendable/retractable fixationhelix assemblies (both electrically active and passive) are known tothose of ordinary skill in the art, and need not be described in greaterdetail here.

The pace/sense electrode (whether a solid tip electrode as describedabove or an active-fixation helix such as shown in FIG. 3) can be madeof any suitable electrically conductive material such as Elgiloy, MP35N,tungsten, tantalum, iridium, platinum, titanium, palladium, stainlesssteel, as well as alloys of any of these materials.

The coil electrodes 255, 257 can take on any configuration suitable fordelivering a relatively high-voltage therapeutic shock to the heart fordefibrillation therapy. In various embodiments, the coil electrodes 255,257 can be made from any suitable electrically conductive material suchas those discussed in the preceding paragraph. The lead 150 alsoincludes a conductor (not shown in FIG. 3) within the lead body 200electrically connecting the coil electrodes 255, 257 to an electricalcontact on the connector assembly 310, which in turn is configured toelectrically couple the coil electrodes 255, 257 to electricalcomponents within the PG 140.

In FIG. 4 is a transverse cross-sectional view of the lead 150 takenalong the line 4-4 in FIG. 3. As shown in FIG. 4, the lead body 200includes an inner tubular member 380 and an outer tubular member 385disposed over and bonded to the inner tubular member 380. The tubularmembers 380, 385 can be made from any number of flexible, biocompatibleinsulative materials, including without limitation, polymers such assilicone and polyurethane, and copolymers thereof. As further shown, theinner tubular member 380 includes a plurality of lumens 390, 395, 400,and conductors 410, 415, and 420 are disposed, respectively, in thelumens 390, 395, and 400. Each of the conductors 410, 415 and 420extends longitudinally within the respective lumen 390, 395, 400, and iselectrically coupled to an electrode (e.g., the electrodes 312, 255, or257 in FIG. 3) and also to an electrical contact of the connectorassembly 310.

In some embodiments, the lead body 200 does not include separate,coaxial tubular members, but instead, includes only a single tubularmember (e.g., the member 380) including one or more longitudinal lumensfor housing the requisite conductors. For illustrative purposes, thethree lumens 390, 395, 400 of the inner tubular member 380 are shownhaving different diameters. In other embodiments, however, the relativedimensions and/or locations of the lumens 390, 395, 400 may vary fromthat shown. In addition, the inner tubular member 380 may include agreater or lesser number of lumens, depending on the particularconfiguration of the lead 150. For example, the inner tubular member 380may include a greater number of lumens to house additional conductorwires and/or electrode coils within the lead 150 for supplying currentto other shocking coils and/or pace/sense electrodes.

In the illustrated embodiment, the conductor 410 is a single layer coilconductor such as would be used in conjunction with a conventional,low-voltage pace/sense electrode, e.g., the tip electrode 312. In someembodiments, for example, the coil conductor 410 is configured to have arelatively high impedance when exposed to electromagnetic energy such asthat present during an MRI scan. In various such embodiments, the coilconductor 410 is configured according to the various embodimentsdescribed, for example, in U.S. Patent Application Publication No.2009/0198314, which is incorporated herein by reference in its entirety.The increased impedance aids in reducing the energy dissipated into thesurrounding body tissue at or near the lead electrode(s). In variousembodiments, the conductor 410 and the lumen 390 are omitted.

As explained in further detail below, the conductors 415 and 420 aremulti-layer conductor assemblies incorporated into the lead 150 toprovide a conditionally-safe MRI-compatible lead design, as well as toprovide improved fatigue resistance and other mechanical propertiesduring delivery and under chronic operating conditions. In variousembodiments, the multi-layer coil conductors 415, 420 include multiplecoil layers having selectively controlled coil properties (e.g., pitch,outside diameter, filar thickness, etc.) to make a highly inductive,highly conductive, small diameter conductor that has suitable mechanicalcharacteristics for a stimulation/sensing lead body.

In the illustrated embodiment, the coil conductor 415 is a three-layerconductor and the coil conductor 420 is a two-layer conductor. Invarious embodiments, multi-layer coil conductors utilizing more thanthree coil layers can be utilized. In various embodiments, the singlelayer coil conductor 410 can be replaced with a multi-layer coilconductor similar or identical to the coil conductor 415 and/or 420.

FIG. 5 is a more detailed side view of the coil conductor 420 accordingto one embodiment of the present invention. As shown in FIG. 5, themulti-layer coil conductor 420 is a three-layer coil conductor 420including an outer coil layer 422, a middle coil layer 425 and an innercoil layer 430. The outer coil layer 422 is disposed about the middlecoil layer 425 which is disposed about the inner coil layer 430. Theouter, middle, and inner coil layers 422, 425, 430 are electricallycoupled to one another in parallel at least at their the proximal ends(i.e., at or near the connector assembly) as well as at their distalends (i.e., at the electrode 255), so as to provide parallel conductivepaths between the connector assembly 310 and the electrode 255. Invarious embodiments, the parameters of the coil layers 422, 425 and 430are configured such that the multi-layer coil conductor 420 has amaximum DC resistance of about 3.0-3.5 ohms, and a high impedance whenexposed to an external alternating magnetic field characterized byfrequencies associated with MRI scans. It will be appreciated that thetwo-layer coil conductor 415 can be configured in substantially the samemanner as the coil conductor 420 shown in FIG. 5 while only includingtwo coil conductor layers.

In some embodiments, the outer layer 422 of the coil conductor 420 canbe wound to coil diameter D_(o) of less than about 0.013 inches. In someembodiments, the outer layer is close pitched and can have one or morefilars having a maximum filar thickness of about 0.004 inches. Inaccordance with various embodiments, the outer layer may or may not bepresent depending on the desired application of the lead. For example,in some cases, the outer layer may be used to change the resistance ofthe multi-layer coil conductor 420. In at least one embodiment, theouter layer 422 may have a filar thickness of from about 0.001 to about0.004 inches.

The middle layer 425 of the coil conductor 420 is close pitched and canbe wound to a coil diameter D_(m) less than the inner diameter of theouter coil layer 422. In some embodiments, the middle layer can have oneor more filars having a maximum filar thickness of from about 0.0007 toabout 0.003 inches.

The inner coil layer 430 of the coil conductor 420 is close pitched andcan be wound to a coil diameter D_(i) less than the inner diameter ofthe middle layer 425. In some embodiments, the inner layer can have oneor more filars having a maximum filar thickness of from about 0.0007 toabout 0.001 inches.

In various other embodiments, multi-layer coil conductor 420 utilizesdifferent ranges of dimensions and other parameters (e.g., filar count)of the respective coil layers 422, 425, 430 depending on the operationalneeds for the lead 150.

In the various embodiments, the filar material can be any suitablematerial exhibiting the desired electrical and mechanical properties. Inone embodiment, the filars of the outer, middle, and/or inner coillayers 422, 425, 430 are made of drawn filled tube (DFT) wire includinga silver core (about 40%) with an MP35N or tantalum cladding. In anotherembodiment, the outer, middle, and/or inner coil layers 422, 425, 430are made of DFT wire including a tantalum core with a MP35N cladding.The coil layers 422, 425, 430 may be comprised of the same or differentmaterials, and each of the coil layers 422, 425, 430 may includedifferent silver fill levels.

In various embodiments, the filars of each layer of the coil conductor420 are wound in the same pitch direction. That is, the individualfilars of each coil layer are wound to have either a right-hand pitch ora left-hand pitch when viewing the coil along its longitudinal axis.

In various embodiments, one or more of the coil layers 422, 425, 430 hasa variable coil pitch along its length, which operates to de-tune thecoil conductor 420 to reduce the effect of externally appliedelectromagnetic radiation (i.e., due to an MRI chest scan).

In various embodiments, the individual coil layers 422, 425 and 430 canbe separately optimized to each exhibit a different inductance, e.g., bymodifying the filar thickness, pitch, and/or coil layer diameter, tofurther modify the performance of the lead 150 under MRI conditions.

Some embodiments of the present invention include one or more layers ofinsulation between one or more of the adjacent coil layers 422, 425 or430. Alternatively, or additionally, in various embodiments, theindividual filars or the coil layers 422, 425, and/or 430 areindividually insulated. Any suitable insulation material can beutilized, if desired. Exemplary insulating materials for the filarsand/or between the coil layers include ethylene tetrafluoroethylene(ETFE), polytetrafluoroethylene (PTFE), expanded PTFE (ePTFE), silicone,and the copolymers of the foregoing.

The various embodiments of the lead 150 described above, advantageouslyminimize induced currents in the lead conductors resulting from exposureto external MRI electromagnetic fields. This is in contrast toconventional lead systems utilizing stranded cable conductors totransmit the shocking currents from the PG to the shocking electrodes.While such cable conductors provide excellent electrical performance fordelivering anti-tachycardia therapy, stranded cable conductors also havea low impedance and thus are susceptible to generation of inducedcurrents when exposed to an alternating electromagnetic field such asthat present during an MRI scan. Various embodiments of the presentinvention can result in lead 150 exhibiting a temperature rise whenexposed to MRI radiation of approximately half that experiencedutilizing traditional high energy cables which are conventionallyutilized to supply energy to high-voltage shocking coils indefibrillation leads. The high impedance conductor configurations forthe lead 150 described above minimize the effects of MRI radiation whilestill providing suitable electrical performance for use inanti-tachycardia therapy applications.

In addition, In accordance with some embodiments, the design parametersof the multi-layer coil conductors can be tuned to control themechanical properties, e.g., effective spring constant and thusstiffness of the overall lead assembly, bending stiffness, and the like.Such tuning may enable the user minimize stresses (loads) on adjacent(distal) polymer components, minimize the potential for shear bondfailure, facilitate/control the lead bias shape, and minimize the impactof axial length tolerance stack-up during assembly, in both low and highvoltage lead applications. For example, in various embodiments, theshocking coils 255, 257 (see FIGS. 2A and 3) are omitted, and the lead150 includes only one or more low-voltage pace/sense electrodes such asthe tip electrode 312 and additional ring electrodes along the lead 150.In such embodiments, multi-layer coil conductors such as the coilconductors 425 and/or 420 can be utilized to provide sufficientelectrical performance in a low-profile design and also optimalmechanical characteristics such as axial stiffness relative to that ofadjacent insulative components such as the lead body 200.

In various embodiments, the multi-layer coil conductor 415 and/or 420can be tuned to control the effective spring constant (expressed inforce per unit length) of the coil conductor 415 and/or 420 so as toreduce or eliminate the difference in axial stiffness between theconductor coil and the adjacent insulating elements (i.e., lead body 200components). By controlling and optimizing the coil conductor springconstant/axial stiffness in this way, the overall axial strength of thepolymeric lead body components can be maintained, for example, where thecoil conductor 415, 420 terminates proximal to the lead body component.Additionally, by reducing or eliminating large differences in the axialstiffness of the conductors (i.e., by utilizing the multi-layer coilconductors 415 and/or 420) and the parallel polymeric lead bodycomponents, shear forces between these elements are advantageouslyreduced. Utilizing the multi-layer coil conductors 415, 420 in lieu ofconductor cables also eliminates snaking of the conductor that canresult due to the contraction and release of a tensile axial load on theconductor.

Exemplary design parameters that can be varied to tune/optimize themechanical characteristics discussed above can include, withoutlimitation, the selection of conductor materials, number of layers, windgeometry, pitch, filar diameter, number of filars, and others.

In addition to the coil conductor configurations described above, thevarious embodiments of the lead 150 of the present invention mayoptionally incorporate other features or techniques to minimize theeffects of MRI radiation. For example, in some embodiments, shieldingmay be added to the lead 150 to further reduce the amount ofelectromagnetic energy picked up from the lead 150. For example, theenergy picked up from the shielding can be coupled to the patient's bodyalong the length of the lead 150, preventing the energy from coupling tothe lead tip. The transfer of intercepted energy by the shielding alongthe length of the shielding/lead can also be inhibited by dissipatingthe energy as resistive loss, using resistive material for the shieldingconstruction.

Various modifications and additions can be made to the exemplaryembodiments discussed without departing from the scope of the presentinvention. For example, while the embodiments described above refer toparticular features, the scope of this invention also includesembodiments having different combinations of features and embodimentsthat do not include all of the described features. Accordingly, thescope of the present invention is intended to embrace all suchalternatives, modifications, and variations as fall within the scope ofthe claims, together with all equivalents thereof.

1. An implantable electrical lead, comprising: a flexible body having aproximal region including a proximal end, a distal region, and alongitudinal body lumen extending from the proximal end through thedistal region; a connector assembly configured to mechanically andelectrically connects the implantable electrical lead to an implantablepulse generator; an electrode coupled to the flexible body in the distalregion; and a multi-layer coil conductor extending within thelongitudinal body lumen from the connector assembly to at least theelectrode, the multi-layer coil conductor including: a first coil layerincluding one or more filars wound so as to have a close pitch; and asecond coil layer disposed about the first coil layer including one ormore filars wound so as to have a close pitch, the filars of the secondcoil layer wound in the same pitch direction as the filars of the firstcoil layer; and wherein the first and second coil layers areelectrically coupled to one another and configured to provide parallelconductive paths between the connector assembly and the electrode suchthat the multi-layer coil conductor has a maximum DC resistance of about3 to 3.5 ohms.
 2. The implantable electrical lead of claim 1, whereinthe filars of the first coil layer and the second coil layer areindividually electrically insulated.
 3. The implantable electrical leadof claim 1, wherein the one or more filars of the first coil layer areelectrically uninsulated from one another, and wherein the one or morefilars of the second coil layer are electrically uninsulated from oneanother, and the lead further includes a layer of insulating material atleast partially disposed between the first and second coil layers. 4.The implantable electrical lead of claim 1, wherein the one or morefilars of the first and second coil layers are made from drawn filledtube (DFT) wires including a silver core and an outer cladding of acobalt-nickel-molybdenum alloy.
 5. The implantable lead of claim 1,wherein the filars of the first coil layer have a maximum filarthickness of about 0.001 inches, and wherein the filars of the secondcoil layer have a maximum filar thickness of about 0.003 inches.
 6. Theimplantable electrical lead of claim 1, further comprising a third coillayer disposed about the second coil layer including one or more filarswound in a close pitch and in the same pitch direction as the filars ofthe first and the second coil layers.
 7. The implantable lead of claim6, wherein the third coil layer has a maximum filar thickness of about0.004 inches.
 8. The implantable electrical lead of claim 7, wherein aninductance of the first coil layer, the second coil layer and the thirdcoil layer are different.
 9. The implantable electrical lead of claim 1,wherein the first coil layer and the second coil layer have a 1-filar,2-filar, 3-filar, or 4-filar construction.
 10. The implantableelectrical lead of claim 1, wherein the first coil layer has a filarthickness of between, and including, 0.001 inches and 0.002 inches. 11.The implantable electrical lead of claim 1, wherein the first coil layeris formed from a 0.0007 inches drawn filled tube (DFT) wire with a 1 to4 filar construction and an outside diameter of the first coil layer isless than 0.004 inches.
 12. The implantable electrical lead of claim 11,wherein the second coil layer has a filar thickness between, andincluding, 0.0007 inches to 0.003 inches.
 13. An implantable medicallead, comprising: a flexible body having a proximal region including aproximal end, a distal region, and a longitudinal body lumen extendingfrom the proximal end through the distal region; a connector assemblyconfigured to mechanically and electrically connect the implantablemedical lead to an implantable pulse generator; an electrode coupled tothe flexible body in the distal region; and a multi-layer coil conductorextending within the longitudinal body lumen from the connector assemblyto at least the electrode, the multi-layer coil conductor including: aninner coil conductor having an outer diameter (OD), a close pitch, andan inner coil pitch direction of clockwise or counterclockwise; a middlecoil conductor having a close pitch, a middle coil pitch direction thesame as the inner coil pitch direction, an inner diameter (ID) greaterthan the OD of the inner coil conductor, and wherein the middle coilconductor radially surrounds the inner coil conductor along the lengthof the inner coil conductor; and an outer coil conductor radiallysurrounding the middle coil conductor having a close pitch and an outercoil pitch direction the same as the inner coil pitch direction; andwherein the inner coil conductor, the middle coil conductor, and theouter coil conductor are electrically coupled in parallel resulting in aDC resistance of approximately 3.5 ohms.
 14. The implantable medicallead of claim 13, wherein the medical lead further includes one or morelayers of insulating material surrounding one or more of the inner coilconductor, the middle coil conductor, or the outer coil conductor. 15.The implantable medical lead of claim 13, wherein the inner coilconductor, the middle coil conductor, or the outer coil conductor have a1 to 4 filar construction and each having a different pitch.
 16. Theimplantable medical lead of claim 13, wherein the inner coil conductorhas a filar thickness of 0.001 inches or less.
 17. The implantablemedical lead of claim 16, wherein the inner coil conductor is formedfrom a 0.0007 inches drawn filled tube (DFT) wire with a 1-4 filarassembly and an OD of less than 0.004 inches.
 18. The implantablemedical lead of claim 17, wherein the outer coil conductor has a wirediameter between, or including, 0.0007 inches to 0.003 inches.
 19. Theimplantable medical lead of claim 13, wherein each of the filars of theinner coil layer, the middle coil layer, and the outer coil layer isindividually insulated.